#main:before {
  content: "";
  display: block;
  background-position: center bottom;
  background-size: cover;
  position: relative;
  /* z-index: -1; */
  height: 380px;
  background: url("https://ce.sysu.edu.cn/bisclab/sites/default/files/%E4%BA%BA%E6%89%8D%E5%9F%B9%E5%85%BB.jpg") center center no-repeat; 
}

@media screen and (max-width: 768px) {
  #main:before {
    height: 240px;
  }
}